Not applicable.
Not applicable.
A particle source in general, and in particular to a particle source is provided for the production of excited particles in gaseous media.
Ion sources play an important part today in many regions of physics and in industrial application (plasma deposition, implantation, ion etching of microstructures, etc.). The requirements on such ion sources are most manifold, e.g., a given kind or charge state of ions, high intensity, high brilliance, pulsed operation, etc. In general the aim is, however, to combine high intensity with good brilliance in ion sources.
In known ion sources, the ions are produced in plasmas, which as a rule are ignited and operated in the sub-millibar pressure region. It is then found that because of this limited gas and plasma density, only ion beams with surface densities of up to about 0.5 Ampere/cm2 can be attained. A detailed description of such known ion sources is to be found, e.g., in B. Wolf, Handbook of Ion Sources, CRC Press, Boca Raton (1995) or I. G. Brown, The Physics and Technology of Ion Sources, John Wiley and Sons, New York (1989), which are wholly incorporated herein by reference.
The brilliance or emittance of the ion beam is limited by the temperature of the ions in the source. This temperature is typically several thousand degrees Celsius for the known ion sources, which corresponds to an energy uncertainty in all three spatial directions of about 0.1-1 eV (electron volt). In order to produce ion beams with high current, usually large plasma volumes are required. The same holds for particle sources for the production of particles, e.g., atoms or molecules with electrons in bound excited states, as for ion sources.
Thus, for example, beams with atoms in bound excited states are used for lithography. This also is a field of application for the present invention.
Therefore an object of the invention is to provide a particle source for excited particles, the particle source having a very small volume, a high particle current, a low emittance and/or a high brilliance, in particular a low energy uncertainty.
A further object of the invention is to provide a cost-efficient and compact particle source for excited particles.
A further object is to make available a particle source for large-area excited particle beams.
The object of the invention is achieved in a surprisingly simple manner by a particle source having a partition with at least one opening. The opening connects a first volume on a first side of the partition with a second volume on a second side of the partition. First particles move from the first volume through the opening into the second volume. Energy is transmitted to the first particles and at least some of the first particles transform to excited states.
In the sense of the invention, the concept xe2x80x9cexcited particlexe2x80x9d includes both particles with electrons in excited bound states and also particles with electrons in excited continuum states, i.e., ions. The concept xe2x80x9cparticle source for the production of excited particlesxe2x80x9d thus includes in particular an ion source and also a source for particles, e.g., atoms or molecules, in bound excited states. The latter can additionally also be ionized. Furthermore the concept xe2x80x9cexcited particlexe2x80x9d in the sense of the invention also includes chemical radicals, e.g., by means of a dissociation, particularly of molecules. The particles are thus in particular to be carriers of potential energy. The particles are excited in a manner such that potential energy is stored and can be transferred in a reaction, e.g., to other particles. The particles can however also be carriers of kinetic energy.
The particle source according to the invention produces in an advantageous manner a directed and cold beam of, or at least with, excited microscopic particles.
As a development of the invention, the particle source or ion source includes a first and a second gas volume on a first or second side of a partition, wherein a pressure difference exists between the first and second gas volumes, and gas flows out of the first into the second gas volume through at least one opening in the partition and when flowing through is ionized or excited in a gas discharge. In particular, the particles, atoms or molecules of gas are electronically excited or dissociated. Thus, by means of the particle source according to the invention, e.g. helium ions or electronically excited metastable states, in particular of helium atoms, can be produced, or radicals, e.g., oxygen radicals, can be produced by dissociation of O2 molecules.
As a development of the invention, the particle source uses a partition comprising a dielectric or electrically insulating base layer, an electrically conductive first layer on the first side of the base layer, and an electrically conductive second layer on the second side of the base layer.
Such partitions, particularly in the form of a flexible foil, can be produced easily and at low cost. A voltage can be applied between the two electrically conductive layers providing extremely high electric field strengths within the small opening due to the small geometry. The electric field strengths in the region of the opening are at least about 104, 105, 106, 107, or even 108 V/cm. For this purpose, only relatively low voltages are required, of the order of about 1-1,000 volt. Because of the high field strengths, the particle source can be operated at high pressures of up to 10xe2x88x923, 10xe2x88x922, 10xe2x88x921, 10, or 102 bar on the first side of the partition. The pressure on the second side of the partition is preferably 10xe2x88x924, 10xe2x88x925, 10xe2x88x926, 10xe2x88x927 or 10xe2x88x928 bar.
As a development of the invention, the pressure difference between the first and second side of the partition is at least one, two, three, four, five or six powers of ten. Thereby the gas expands substantially adiabatic isochorically on flowing through the opening. Thereby the whole enthalpy of the gas in converted into directed motion, so that the gas atoms receive an average speed of v=(5 kT/m)1/2, where k is Boltzmann""s constant, T is the gas temperature, and m is the particle mass. The gas then cools to temperatures in the milliKelvin region. An ultrasonic gas jet arises. Ultrasonic gas jets are basically known to a skilled person. The ultrasonic gas jet is now ionized by electron impact ionization in the region of the opening, according to the invention, so that an extremely cold and directed particle beam or ion beam arises.
As a development of the invention, the coldest inner portion of the particle beam is stripped out by a diaphragm, an aperture or a skimmer, so that an even lower energy uncertainty is produced. In order to achieve particularly low particle temperatures, the gas in the first volume is preferably cooled to below 100, 70, 30, 20 or 10 Kelvin.
As a development of the invention, the operation employs a mixed gas of a carrier gas and a working gas, where preferably only the atoms or molecules of the working gas are ionized. The carrier gas substantially determines the thermodynamic properties of the gas expansion. For example, helium is particularly well suited as the carrier gas because of its low atomic weight and its high excitation potential and ionization potential; it cools during the expansion of the working gas. Furthermore, in helium, because of its high electronic excitation energy, the electrons arising in the gas discharge and thereafter accelerated by the electric field assume a high kinetic energy in spite of the high gas pressure. The working gas has a substantially lower excitation potential and ionization potential than helium, so that substantially only the working gas is excited and/or ionized. By selection of the mixing ratio of the carrier and working gases, the average kinetic energy of the electrons, and hence the excitation and/or ionization of the working gas, can be adjusted in a targeted manner.
The transverse momentum uncertainty of the gas, and thereby of the particle beam, is further reduced by cooling the carrier gas, so that the particle beam has an extremely good transverse brilliance.
A development of the invention uses so-called microstructure electrode foils. A microstructure electrode (MSE) comprises one, plural, or many micro-openings. In the case of plural or many openings, these are preferably arranged as a regular, two-dimensional matrix. This can be produced cost-efficiently, over large surfaces, with a small distance between the openings and very small openings. In this embodiment, a large-surface plasma is produced by means of a great number of pores. Ion current densities can thereby be produced of at least 10xe2x88x923, 10xe2x88x922, 10xe2x88x921, 10, 100 or 1,000 Ampere/cm2 in a continuous current or in pulsed operation.
The invention is described hereinbelow with the aid of preferred embodiments and with reference to the accompanying drawings.
Ion sources according to the invention are presented by way of example in what follows. It is however evident to the skilled person that particles, particularly atoms or molecules with electrons in bound excited states which arise, e.g., by electron impact excitation or electron capture can also be produced with the ion sources shown.